First-half tries from powerful centre Ben Johnston and skipper and No.8 Hugh Vyvyan, all of which were converted by fly-half Dave Walder, put England into a useful 17-6 half-time lead, with the visitors scoring another four tries in the second half.
Back rower Pete Anglesea, inside back Henry Paul, mobile hooker Phil Greening and starting fullback Iain Balshaw scored England's second-half tries, with Walder kicking another two conversions and Olly Barkley adding one conversion.
However, it wasn't plain sailing early on, with flanker Kort Schubert signalling USA's ambition in an early charge to the posts. That resulted in a Link Wilfley penalty and Johnston finally put England on the scoresheet in the 24th minute. The Saracens centre was first to Walder's chip over the Eagles' defence for an easy run-in.
Walder converted and then traded penalties with Wilfley for a 10-6 lead before Vyvyan picked up the ball from Paul's long pass and galloped in.
Pete Anglesea scored England's third try in the second half when he crashed over next to the flag from James Simpson-Daniel's pass and Walder's extras gave them some daylight at 24-6.
The Newcastle Falcon, relishing some game time after missing most of the season with a broken leg, created Paul's try with a dribbled drop-out on his own 22, Vyvyan provided the link and the Gloucester centre beat David Fee to the corner.
There was still time for Phil Greening and Martyn Wood to employ a one-two at the front of the line-out for the hooker to go over, while Iain Balshaw had the last word with a try from Simpson-Daniel's feed.
"English rugby is on a real high at the moment," said Paul after the match. "It doesn't matter whether you're in New Zealand and Australia or over here – it's all about playing well as a squad.
"We have done a job here and hopefully we will be good enough if we have to step up. It's good that England has a pool of around 60 players to chose from rather than just 20 or so."
Paul also saluted Mallinder and assistant Steve Diamond, saying: "The coaching staff have really fired us. We have learnt a lot from Jim and Steve and they have done a good job of keeping everyone tight."
England now travel to Tokyo for two games against a Japan Select XV (Thursday, July 3) and Japan (Sunday, July 6) and for Mallinder the job is only half done.
"Three from three – you can't ask for anything more than that but we set out our goals from the outset and that's winning all five games; that is our No.1 priority. We have got two tough games in difficult humid conditions; we don't know much about Japan so the challenge is there for us."
For the USA Eagles:Pens: Wilfey 2
For England 'A':Tries: Johnston, Vyvyan, Anglesea, Paul, Greening, BalshawCons: Walder 5Pen: Walder
Yellow card(s): Kain Cross (USA Eagles, 42), Chris Jones (England, 69), Luke Gross (USA Eagles, 69)
USA Eagles: 15 Link Wilfley, 14 David Fee, 13 Philip Eloff, 12 Kain Cross, 11 Riaan van Zyl, 10 Matt Sherman, 9 Kimball Kjar, 8 Dan Lyle (captain), 7 Conrad Hodgson, 6 Kort Schubert, 5 Luke Gross, 4 Brian Surgener, 3 Dan Dorsey, 2 Kirk Khasigian, 1 Mike MacDonald.Replacements: 16 Mark Griffin, 17 John Tarpoff, 18 Jurie Gouws, 19 Olo Fifita, 20 Kevin Dalzell, 21 Jason Keyter, 22 Mose Timoteo.
England 'A': 15 Iain Balshaw, 14 Mark Cueto, 13 Ben Johnston, 12 Henry Paul, 11 James Simpson-Daniel, 10 Dave Walder, 9 Martyn Wood, 8 Hugh Vyvyan (captain), 7 Dan Hyde, 6 Pete Anglesea, 5 Alex Codling, 4 Alex Brown, 3 Will Green, 2 Andy Titterrell, 1 Andrew Sheridan.Replacements: 16 Phil Greening, 17 David Flatman, 18 Mike Worsley, 19 Paul Volley, 20 Chris Jones, 21 Olly Barkley, 22 Phil Christophers.